In hole drilling or slot machining, it has been discovered that the essentially incompressible jet of the abrasive water jet and the abrasive slurry jet (AWJ/ASJ) builds up an extremely high piercing pressure at the bottom of the blind hole or slot (hereafter referred to as the cavity) before break through. The piercing pressure build up is a direct consequence of deceleration and reversal of the AWJ as the bottom of the cavity is approached. For delicate target materials such as composites and laminates, surface/subsurface damages and delamination may result when the piercing pressure exceeds the tensile strength of the materials or the binding strength of the adhesive of the laminates. Furthermore, the large difference in the density between the water and abrasives lead to a lag of the abrasives' trajectories behind the streamline of the water as the return slurry turns around and reverses its course at the bottom of the cavity. In the return slurry, the spent abrasives that still possess considerable erosive power are forced toward the wall of the cavity, particularly near the cavity entrance where the slurry exits. As a result, the spent abrasives (typically 12% by weight and 3% by volume) are forced toward the wall of the cavity and induce excessive wear on the wall near the cavity entrance, leading to nonuniformity in the hole diameter.
Recent development of abrasive slurry jets or abrasive suspension jets (ASJ) by directly pumping an abrasive slurry through a nozzle has further improved the erosive power the UHP technology. It has demonstrated that under identical hydraulic and abrasive conditions, the two-phase ASJ consisting of water and abrasives has erosive power up to five times higher than that of the three-phase AWJs consisting of water, air, and abrasives. Evidently, the momentum transfer from the ultrahigh-speed water is more efficient in the ASJ with direct pumping of the slurry than in the AWJ with entrainment of abrasives downstream of the jet orifice. At present, the maximum pressure used in commercial ASJ systems is limited to 15,000 to 20,000 psi (103 to 138 MPa) due to lack of materials capable of resisting the erosive power of the ASJ at pressures higher than the above range. With the advent of development of advanced materials, ASJs operating at pressure comparable to that of AWJs are expected to become a superior machine tool to AWJs for various applications. However, the ASJ would be more problematic than the AWJ in terms of surface/subsurface damage. Because of the lack of entrained air in the two-phase slurry of the ASJ, the ASJ jet material will be less compressible than that of the three-phase slurry of the AWJ, creating still higher piercing pressures because they are proportional to the incompressibility of the fluid inside a blind cavity. Therefore using flash vaporization of the jet is even more effective in an ASJ than in an AWJ for mitigating surface/subsurface damage of delicate materials.
For hydroscopic materials where the use of water jets is undesirable or unacceptable, a UHP abrasive cryogenic jet (ACJ) using liquefied nitrogen (LN2) as the working fluid has been developed for coating removal and machining advanced/delicate materials. One of the key differences of AWJs/ASJs and ACJs is that the LN2 in ACJs changes phase after exiting the mixing tube whereas water in AWJs/ASJs does not. When drilling holes or slots into a target material to form a cavity, the cavity size increases with time by the erosive action of the abrasives. As the ACJ jet is entering the cavity, the N2 gas evaporated from the liquid N2 escapes easily from the cavity. As a result, the piercing pressure of the ACJ inside the cavity is considerably weaker than that of the AWJ/ASJ. Surface/subsurface damages are mitigated provided the reduced piercing pressure is weaker than the tensile strength of the materials or the binding strength of the adhesive of the laminates. As the LN2 entering the cavity continues changing into N2, the return flow consists mostly of dry abrasives and gas instead of a slurry as in the AWJ/ASJ. In other words, the return flow is considerably less organized and coheres less for the ACJ than for the AWJ/ASJ. The trajectories of the return spent abrasives in the ACJ are random in nature as they collide with the incoming abrasives and the side wall on their way out. The benefits of the phase change of the working fluid are therefore to mitigate surface/subsurface damage by reducing the piercing pressure inside the cavity and minimize nonuniform secondary damage by transforming the return flow from an abrasives slurry with liquid to dry abrasives and gas.
Although the advantages of ACJs over AWJs/ASJs for machining delicate materials have been demonstrated, there is considerable trade off in terms of economical and technical issues to be overcome before ACJs can be commercialized as a machine tool. ACJs are bulky, expensive to maintain, and difficult and hazardous to operate. First of all, the LN2 requires a very large cryogenic storage and delivery facility. To ensure that no phase change takes place inside the UHP pump, an inline subcooler is often required just upstream of the pump to lower the temperature of the LN2. The cryogenic temperature presents an extremely hostile environment to components such as the seals and valves of the pump and significantly reduces their operating life. Equally import, the spent LN2 and N2 must be vented properly to prevent unacceptable dilution of the O2 in the work space.